Authorities in the southern Chinese city of Guangzhou are holding an activist under criminal detention after he joined a protest last weekend in support of political reforms, a Hong Kong-based rights group said.

Xiao Yong was taken into custody on Tuesday for being part of an "illegal gathering," which fellow activists said was likely connected to a demonstration in Guangzhou on Saturday, the China Human Rights Defenders (CHRD) said in an e-mailed statement.

Police are also holding fellow activist Huang Wenxun, who took part in the same demonstration in response to calls for unspecified "political reforms" by outgoing premier Wen Jiabao at the annual parliamentary sessions in Beijing last month, the group said.

The demonstration also called on political leaders to publicly disclose their personal assets.

Xiao's wife was handed a detention notice issued by the Guangzhou public security bureau on the day he was detained, and police also searched the couple's home, confiscating his computer, CHRD said.

Xiao is currently being held at the Guangzhou No. 1 Detention Center, where he has been refused permission to see visiting lawyer Wang Yajun on the grounds that his case is "confidential," the group said.

Xiao's wife has been allowed to drop off clothes and money for her husband, but has also been denied permission to visit him.

Authorities insisted to Wang that, during the investigative stage, only those handling Xiao’s case can approve visits, since the case is considered “confidential.”

Online petition

Xiao's detention has prompted an international, online petition calling for his release.

U.S.-based rights activist Ge Xun, who was himself detained and interrogated by state security police on a visit to Beijing in February for his mother's funeral, said he had started the Twitter-based petition because he was "very, very angry" at Xiao's detention.

"Premier Wen Jiabao himself made special mention of political reforms at his parliamentary press conference," Ge said. "They were just helping the government to ... spread its message, and they arrested him."

"They didn't do anything to break the law, so this is really unacceptable."

Ge said it was still unclear which level of China's ruling Communist Party had ordered Xiao's detention.

"Now it hardly seems to matter whether you support the government or are against it," he said. "If you have any social conscience and you want to actually do something, they will arrest you."

"This is ridiculous and absurd," Ge said.

Ge's petition had garnered 193 signatures by Friday, suggesting that some netizens inside China had been unable to get around official Internet blocks and filters known as the "Great Firewall."

"A lot of people haven't been able to get around the Wall," Ge said. "They have e-mailed me asking me to sign it on their behalf."

Twitter and Facebook remain blocked in China, although similar services run by Chinese Internet companies are massively popular.

Reported by CK for RFA's Mandarin service. Translated and written in English by Luisetta Mudie.